Numerous control circuits have been designed to apply a voltage or current to an electrical load after a time delay. Examples of such circuits are disclosed in U.S. Pat. No. 3,745,382 to Hoge et al., U.S. Pat. No. 3,597,632 to Vandemore, and U.S. Pat. No. 3,764,832 to Stettner. However, these and other known control circuits are relatively complicated and have numerous components, thus increasing manufacturing difficulty and costs. Further, these and other known control circuits typically provide relatively lengthy time delays, on the order of five minutes, and are unreliable.
Control circuits are used in a variety of applications including, for example, to activate an alarm circuit in a fire protection system. Conventional fire protection systems typically include a source of water or other fire-extinguishing fluid, a detector for detecting the flow of the fire extinguishing fluid through a pipe or conduit, and an alarm circuit or other load that is activated when a sufficient flow is detected.
In such systems, the alarm is preferably not activated immediately upon detection of fluid flow in the conduit, because flow may occur due to a "water hammer" or fluid backwash within the system. If the alarm were activated immediately upon detection of a water flow, a large number of false alarms would result.
In order to reduce or eliminate such false alarms, a control circuit can delay the activation of the alarm for a predetermined time following detection of an alarm condition. Early detection and control circuits included simple mechanical devices, such as dashpots in which air was forced into and out of a chamber. The alarm would not sound until the air was completely out of the chamber, at which time a switch would close to activate the alarm.
These and other conventional detection mechanisms were designed to provide a delay in the range of 30 seconds to 90 seconds. However, these devices were unreliable and inaccurate, and were thus unsuccessful in eliminating false alarms. Accordingly, solid state electrical load control circuits were developed for fire protection systems such as the time delay circuit known as ICM/HMKS-W1104. These electrical load control circuits delay activation of the alarm until an electrical sensor or switch is rendered conductive.
It would be desirable to provide a relatively simple, reliable, and easy-to-install sensor circuit with minimal current draw, in order to detect a condition (such as fluid flow) which requires activation of a load such as an alarm. While certain flow sensing devices are known, such as those described in U.S. Pat. No. 3,749,864 to Tice and U.S. Pat. No. 4,791,254 to Polverari, these and other similar devices include relatively complex arrangements of moving parts.